1. Field of the Invention
The present invention relates generally to the construction of flexible, torque-transmitting elements, and more particularly to the construction of a drive cable for rotation of an ultrasonic transducer for use in vascular catheters.
Intravascular imaging of blood vessel lesions prior to percutaneous transluminal angioplasty, atherectomy, and other interventional procedures, promises to be of great benefit. A particularly successful design for an intravascular imaging apparatus employs a rotatable ultrasonic transducer, where the transducer is attached to the distal end of a flexible drive cable. The transducer may be rotated within a catheter body or a sheath in order to transmit an ultrasonic signal and produce a video image by well-known techniques.
Although very successful, the need to reduce the size of drive cable to very small diameters (to be compatible with introduction into very small coronary arteries) presents a number of technical challenges. In addition to the very small diameter, the drive cable should be highly flexible so that it can pass through tortuous regions of the vasculature, particularly the coronary arteries. It is also very important that the drive cable provides uniform rotation along its entire length, i.e. avoid rotational wind-up which can cause variations in the rotational speed of the transducer and resultant image distortion.
The construction of transducer drive cables for intravascular ultrasound devices is further complicated by the desire to run transducer lead wires through a lumen in the cable itself. Such designs avoid an increase in an effective diameter which would result from placing the leads on the surface of the drive cable. Coaxial lead cables have been utilized in the past and have the added advantage that they provide radiofrequency (RF) noise shielding. Coaxial cables, however, suffer in that they are generally large and difficult to use in the smallest intravascular devices, e.g. coronary devices.
Twisted pairs of insulated electrical wire have also been used as lead wires in the drive cables of ultrasonic imaging catheters. Such twisted pairs are advantageous since they are available in very small diameters, but problematic in that they can contribute to non-uniform torque-transmission properties of the drive cable. Such wires are typically formed from malleable metals, such as copper and silver, which can deform during storage and when the drive cable is bent during use. Such malleable metals are not highly resilient and do not tend to return to their initial, straightened condition, as is the case with the body of the flexible cable. Thus, deformed lead wires within the outer cable body can create slight bends and other non-uniformities within the entire drive cable assembly. If the drive cable is not perfectly straight, it cannot spin uniformly in a tortuous vessel and will preferentially favor certain configurations within the vessel as it is rotated. In some rotational positions, the drive cable will be in a relaxed state, while in other positions more resistance will be met due to the force required to straighten out the combined drive cable and internal lead wires. Such non-uniformities can result in non-uniform rotation of the drive shaft, which is disadvantageous for the reasons discussed above.
A second problem with twisted pairs of electrical lead wires within such drive cables results from their lack of shielding. The signals transmitted by such twisted pairs are subject to greater degradation than that experienced by signals traveling through a coaxial cable. Additionally, the dielectric constant surrounding the twisted wire pairs will change if a liquid comes in direct contact with the wires. Impedance matching of the ultrasonic transducer and twisted pair lead wires is typically accomplished in a dry environment. Many ultrasonic systems require that the drive cable be immersed in a flushing liquid in order to provide acoustic coupling for imaging. Should the flushing liquid penetrate the drive cable body, and thus contact the internal twisted pair of electrical leads, the impedance surrounding the leads can be greatly reduced. Such a reduction will interfere with the impedance match between the transducer/wire system and the display unit.
It would therefore be desirable to provide improved drive cables for ultrasonic transducers and other rotatable electrical sensors. It would be particularly desirable to provide drive cables which can be constructed to have very small diameters and employ a twisted pair of electrical lead wires, where the electrical lead wires do not substantially degrade the mechanical characteristics of the combination of cable and lead wires. Even more particularly, the combination of cable and lead wires should be able to provide for uniform torque-transmission along the entire length of the drive cable in order to produce ultrasonic images having minimum distortion. The drive cables should further provide enhanced RF shielding of the electrical leads and should further provide for sealing of the electrical leads so that the drive cable can be immersed in a flushing liquid without wetting the wire leads.
2. Description of the Background Art
Rotatable ultrasonic transmission core elements having internal signal leads are disclosed in U.S. Pat. Nos. 5,243,988; 5,203,338; 5,199,437; 5,002,059, 4,951,677; and 4,841,977; and WO 92/03095. Catheter guide wires having internal means for rotating an ultrasonic transducer are disclosed in U.S. Pat. No. 5,095,911 and WO 93/16642. Ultrasonic imaging catheters having internal drive cables which can carry signal leads are described in U.S. Pat. Nos. 5,000,185 and 4,794,931. U.S. Pat. No. 5,209,235 describes apparatus for coupling to lead wires from an ultrasonic imaging catheter. U.S. Pat. No. 5,108,411, describes a flexible catheter drive shaft made in part from counter wound helical coils.